Black holes regularly collide and merge in the Universe. Scientists analyzed hundreds of such events and noticed a strange pattern. The most massive black holes formed from the merger of smaller ones in the past.
Hidden information
Every time two black holes spiral closer together and merge, waves arise in spacetime. These oscillations contain information about the mass of each black hole and about its spin — that is, the speed and direction of its rotation.
Pairs of black holes form in two ways. In the first case, two massive stars are born close together, live their lives side by side, and eventually turn into black holes that continue to orbit each other. In the second case, black holes initially exist alone and only later randomly form a pair in dense star clusters.
The mass and spin of each black hole depend on how it formed. Therefore, the two scenarios leave slightly different signatures in the signal detected by the LIGO, Virgo, and KAGRA observatories. For a long time, however, it was not possible to distinguish these differences among hundreds of recorded events.
Two paths to one conclusion
The first study was prepared by a group led by Katelyn Plunkett of the Massachusetts Institute of Technology. The researchers built a model based on two well-measured spin parameters that show how closely a black hole’s rotation is aligned with its orbital motion.
The second study was led by Sharan Banagiri of Monash University in Australia. His team made no prior assumptions and instead allowed statistical analysis to determine on its own how many groups the black holes should be divided into. Both papers were published in the peer-reviewed journal Physical Review Letters, according to Phys.org.
A supermassive population
Despite their different starting points, both teams identified a group of black holes distinguished by unusually large mass. Each such object is at least 40 times more massive than the Sun.
According to Katelyn Plunkett’s conclusions, these giants have fast and chaotically oriented spins. This configuration is characteristic of black holes that formed as a result of earlier mergers, rather than through the collapse of stars.
Sharan Banagiri and his colleagues arrived at a similar threshold value and also detected high spins. However, their analysis did not find a clear sign of origin from earlier collisions, so the authors urge caution in interpretation.
The second generation
Stellar collapse does not create a black hole in the range of roughly 60 to 130 solar masses because of the effect of pair instability, so the supermassive group with chaotic spins points to another path for the formation of such objects.
Together, these results are among the strongest evidence to date for the existence of second-generation black holes. This is what scientists call objects that did not arise from the collapse of a massive star, but from the merger of two predecessors.
This also suggests a possible path for the formation of the supermassive black holes at the centers of galaxies. A chain of repeated mergers could gradually increase their mass step by step.
